The present invention relates to a semiconductor device for signal amplification, in particular, relates to a configuration of a transmission amplifying device for controlling gain of a transmitting signal of a transmitting section in mobile communications equipment.
As a communications system to a mobile terminal etc. in a mobile communications field, there is currently UMTS (Universal Mobile Telecommunications System; European standard communications system: compatible with W-CDMA in Japan). UMTS has spread widely as a communications system of a third-generation digital mobile-phone.
In the UMTS transmission system, a power control range is 86 dB and a high dynamic range is required. In order to respond to such a high dynamic range, gain is usually controlled by plural stages of amplifiers. However, when the number of stages of amplifiers increases, a noise of each amplifier is amplified, resulting in an increased total noise. In a mobile terminal, miniaturization of a package is required for reduction of a device mounting surface area, and removal of a SAW (surface acoustic wave) filter for denoising is required. Such a configuration requires an amplifier to exhibit a characteristic with a severe noise specification.
A variable gain resolution required by a base station is 1 dB at an antenna end. In order to satisfy the requirement, a very fine step of gain, 0.25 dB, is required as resolution inside an RF-IC which generates a high-frequency signal. In the UMTS transmission system, a power consumption is large compared with a receiving system, and it is required that LO leakage (leakage current consumed from a DC power supply at the time of no high-frequency signal input) should be suppressed. As a countermeasure to suppress the LO leakage at the time of a low power output, the gain of a high-frequency unit is usually controlled and the LO leakage is suppressed accordingly.
Patent Document 1 (Japanese Unexamined Patent Publication No. Hei 10 (1998)-70424) discloses an example of a configuration of a variable gain amplifier. In the variable gain amplifier disclosed by Patent Document 1, plural transconductance amplifiers are arranged in series and coupled to an input terminal in common. Each output terminal of the transconductance amplifiers is coupled to a voltage dividing node of an R-2R ladder resistor network via a switching element. The switching element is set to on or off by the corresponding bit of a control word C<N−1:0>. An output terminal of an adjoining transconductance amplifier is coupled via a series resistance R of the R-2R ladder resistor network. The transconductance amplifiers are coupled with constant current sources, respectively, and currents outputted from the transconductance amplifiers are shunted by the R-2R ladder resistor network, and supplied to the output terminal of the amplifier. By the shunt effect of the series-coupled resistor R, a weight of (½) is attached to the output currents of the transconductance amplifiers, and the gain is directly controlled by a total current produced according to a binary digital code of binary digits of the control word C(0), C(1), . . . , C(N−1)=C<N−1:0>.
(Patent Document 1) Japanese Unexamined Patent Publication No. Hei 10 (1998)-70424